Northern prairies saw some songbirds return in the 1990s as farmers shifted cropland into grasses. Crop prices were poor then, so idling land benefited farmers and birds.

Helping songbirds, however, is harder when wheat's topping $7 a bushel.

The rebounding grain market the past few years has led farmers to withdraw land from the federal Conservation Reserve Program and plow it back into crops — slashing the birds' habitat. Now, the birds that returned two decades ago are leaving again, raising concerns about the prairie's long term health.

The western meadowlark, the grasshopper sparrow and the savannah sparrow, "seem to be declining" said Larry Igl, a biologist at the U.S. Geological Survey's Northern Prairie Wildlife Research Center in Jamestown, North Dakota.

"We see that in eastern North Dakota and western Minnesota that those species are starting to decline," he added. "We have landowners in our western Minnesota counties asking us, 'What happened to all the bobolinks?'"

Grassland songbirds might not have the political clout of ducks, pheasants or other game birds, but people who live on the prairie are seeing a change. Landowners are "starting to notice some of these grassland species are disappearing," said Igl, who's been watching grasslands and birds in western Minnesota, eastern North Dakota and Montana since 1990, the longest such research in the country.

Eggs are nestled under thick grass in this well-hidden savannah sparrow nest on a grassland near Glyndon, Minnesota.Dan Gunderson/MPR News

Grassland songbird populations are shrinking generally, but there's no doubt Conservation Reserve Program land has been an important habitat for some species and reserve land has fallen dramatically.

In Minnesota, CRP acreage fell to 1.4 million acres in 2013, down nearly 25 percent from 2006, according to the USDA. It's fallen nearly 50 percent in North Dakota — from 3.4 million acres in 2006 to 1.8 million last year.

That's likely to continue — the 2014 farm bill will cut 8 million acres from the reserve in the next four years.

A combination of habitat being turned into farmland and trees taking over some grassland might account for the songbirds' population decline, Igl said, adding it may take several years to see the full effect of habitat loss.

The CRP's "definitely been beneficial" for some bird species and "as we lose it I'm sure it will have an impact," said Marissa Ahlering, a prairie ecologist for The Nature Conservancy, as she conducted a recent survey in a field just east of Moorhead, part of a multi-year study across several states looking at public land, conservation land and private land used for grazing.

Ahlering identified 18 species in the grassland, including meadowlark and grasshopper sparrow, which sounds a bit like its namesake insect.

Some birds, she said, prefer tall dense grasses, other birds like shorter grasses. "That's the challenge with grassland birds. You need a mosaic of that in order to support all species."

She said she hopes her bird surveys will help change how land is managed. Right now, public lands, private lands and tracts owned by conservation groups like the Nature Conservancy are often managed as individual plots.

"We're going to start to need to think more about coordination of landscape management," she added, "to think about how to create that mosaic of structure across ownerships...not just the land you own."